BLOWER AND MOTOR CALCULATIONS
By Norm Christopherson
This section includes information on the sizing of motor pulleys, changing rpm
and cfm of blowers, motor selection for blowers, belt length changes, and using
fan laws for troubleshooting. Also included is information on motor efficiency
and horsepower calculations as well as several motor calculations for energy
efficiency and electrical troubleshooting.
The Fan Laws Applied
A set of mathematical relationships called the fan laws governs the pulley
sizes, rpm of each pulley, and the quantity of air in cfm. The fan laws are very
useful for practical adjustments on air conditioning and heating systems. Learn
the fan laws and relationships, relate them to the psychrometric chart and a
practical and fascinating troubleshooting aid is available to help solve problems.
Figure 13-1
A typical blower and motor pulley combination is illustrated in figure 131. As illustrated in figure 13-2, a motor drives a motor pulley at 1750 rpm. The v
belt transfers the rotational motion to the blower pulley, which operates a
blower. The blower is used to move a given cubic feet of air per minute through
an air conditioning system.
Figure 13-2
In the above diagram, (See figure 13-2) the two pulley sizes are known and the
rpm of the motor pulley is known. The motor pulley rpm is read from the motor
data plate. Using a ratio the blower rpm can be found.
To solve any ratio such as this one, simply remember to cross multiply and
divide.
In this problem cross-multiply the 1750 times the 8 to get 14000. Divide the
14000 by 16 to get 875 rpm. The RPM of the blower is 875. Using the cross
multiply and divide method, it is impossible to go wrong. It is not possible to
cross multiply the wrong way, as there is an unknown in the formula. Also when
it comes time to divide, there is only one number left to divide by.
This formula is often used when a change in motor pulleys is necessary in
order to make a change in air flow cfm. This formula is first used as used in the
example, to determine what the blower rpm is before any changes are made. This
eliminates the need for an actual rpm measurement on the blower
pulley. Therefore, a tachometer is not necessary however; a tachometer is still
useful to check that any changes made are accurate.
Now that it has been determined that this example system has a blower rpm
of 875, assume that an airflow measurement was taken or that the airflow in cfm
is already known. If it is not known there are several possible methods of
determining the cfm. These methods are covered in the chapter on airflow
measurement.
Assume that this is a 10-ton capacity system as stated by the
manufacturer. Assume that an airflow measurement has determined that the
system is moving 3000 cfm of air. From the chapter on rules of thumb it was
stated that for comfort air conditioning there should be 400 cfm per ton. This is a
10-ton system and should then have 4000 cfm. The system is 1000 cfm short.
A technician has looked for the obvious causes of low airflow. The
evaporator air filter is clean, the evaporator coil is clean, the blower wheel is
clean, the belt is not slipping and no obstructions are in the ductwork. It has
been decided that an airflow adjustment is required.
The goal is to increase the cfm from 3000 cfm to 4000 cfm. Changing a pulley
size or making an adjustment on the motor pulley if it is adjustable will
accomplish this. The most economical and practical pulley to replace is the
motor pulley. The motor pulley is less costly, it is the easy one to change, and as
the smaller of the two pulleys, it probably has the greatest wear on it. The smaller
pulley will always wear the fastest as it is turning more revolutions per minute
and it has a smaller circumference for the belt to spread the wear over.
New RPM
The two existing pulley sizes are now known, the rpms of both of the existing
pulleys are known, and the existing cfm is known. The new desired cfm for the
system is also known. Now the missing information is;
1) What new blower rpm is required to obtain the new cfm?
2) What new motor pulley size is required to get the new blower rpm and cfm?
3) Since the blower will be moving more air, will this overload the motor, and if
it
does overload the motor, what is the new horsepower required?
In this formula the 1 stands for what we have, and the 2 is for what we want.
What we have is 3000 cfm with the blower speed of 875 rpm. What we want is
4000 cfm, and are missing the new required rpm to go with the 4000 rpm. So, the
unknown information is question one above, what new blower rpm is required to
obtain the 4000 cfm? Place the appropriate known values into the formula, crossmultiply and divide, and the answer are provided.
Cross multiply 4000 times 875 to get 3,500,000 then divide that by 3000 to get
1166.67 rpm. The blower rpm required to move 4000 cfm on this system is 1167
rpm. As long as the blower rpm can be set within 10% of the 1167 rpm, the
system will be considered correctly adjusted. There is no reason why better than
10% cannot be achieved. As will be explained fully later, more cfm than
necessary may seem like a benefit however, this is a costly mistake in terms of
both comfort control and operating cost. Correct airflow is always best, no more,
no less.
Now to answer the second question. What new motor pulley size is required
to get the new blower rpm and cfm? This is found by using the first formula
again.
This time the unknown to be solved for is the new motor pulley dia which is
DIA a. Notice that the motor rpm is not changed. The change will be in the motor
pulley size.
Cross multiply 16 times 1167 to get 18,672 then divide by 1750 to get the new
motor pulley size of 10.7". The existing 8" pulley may not be able to be adjusted
to that new diameter, and a new fixed pulley of exactly 10.7" may not be
available. However, a fixed pulley close enough can be selected or a new
adjustable pulley of the correct adjustment range can be selected. These pulley
diameters are not outside diameter measurements, but are to be pitch diameter
measurements. Those not familiar with pulley pitch diameter should read the
section in this book on pulley nomenclature.
The third question now needs to be addressed. Since the blower will be
moving more air, will this overload the motor, and if it does, what is the new
horsepower required?
Motor Horsepower Calculations
As the blower rpm and cfm increase there is an increase in the amount of
work which the motor is required to do. The amount of additional work in
horsepower for even a small increase in rpm and cfm is impressive. Few
technicians realize the large increase in load on a motor with even the smallest
increases on adjustable pulleys.
To continue this example assume that the existing motor nameplate data was
available from the start of the job and is as follows. Remember this is not the
actual work the motor is doing, but is what the data plate states the motor is able
to do.
Existing Motor Name/Data Plate Information
10 HP ,
230 Volts 3 Phase
1750 Rpm,
PF .91,
FLA 23.5
%Eff .88
SF 1.15
This motor information will be used in other example calculations and not all
of the plate data is necessary to solve this particular problem. At this point it is
not important to know what all this information is.
Assume that when the technician measured the pulleys and obtained the
motor information, that a motor running amperage measurement was taken. This
amperage is for the original conditions with a motor pulley of 8", a blower rpm of
875, and the 3000 cfm airflow rate. The running amperage under those conditions
was found to be 16 amps. The 16 amps is a full 7.5 amps under the FLA rating of
the motor. The FLA is the full load amps or the amperage the motor will draw if it
is doing its rated horsepower worth of work. For this motor, if it is doing a full 10
HP work load, it would draw 23.5 amps, it is only drawing 16 amps. This shows
there is more HP available. The motor can do more work before it reaches it's
design limit.
However, exactly how many horsepower is it operating at? Whatever it is, is
there enough to handle the increase from 3000 cfm to 4000 cfm? If not, what new
HP rating will the new motor need?
1) What is the existing operating horsepower? (Called brake horsepower)
What will the new motor horsepower be after increasing the airflow?
The existing motor must be changed. Notice that the horsepower increased
by the cube of the change in rpm. This is significant and important to keep in
mind while servicing systems.
Where rpm is used, the cfm can be substituted in the formula to get the same
answer. Instead of using the new and existing rpm, the new and existing cfm will
work just as well. Use whichever is most convenient, but remember to cube the
number after dividing or the answer will be entirely wrong.
HOW TO CUBE
To cube a number the number is multiplied times itself three times.
The number 3 in the right hand corner of the formula means the number is to be
cubed. If the number is 2 then the cube of two is 8. (2 times 2 times 2 is 8)
To square a number multiply the number times itself two times. The square of
2 is 4. (2 times 2 is 4) A 2 in the right hand corner of a formula means the number
is to be squared. In air conditioning formulas some squaring and cubing are
necessary. Most scientific calculators have special squaring and cubing function
buttons.
So, in this example the existing 10 HP motor will not be large enough to do the
job and the calculated new HP is 16.13. Normally the motor sizes available are 10,
15, and then up to 20 HP. A 20 HP motor should be selected to replace the
existing motor.
A technician able to make these kinds of calculations is able to determine
what system changes may need to be made, how to make them, and know that
they are correct. Using mathematics all guessing is over and the technician
gains confidence in the decisions made.
New Amps Formula
Another useful formula to use before making a motor pulley adjustment is the
new amps formula. Using a new example of a blower motor look at how simple
this is and how useful it will be on the job.
A blower motor is drawing 8 amps. The FLA rating of the motor is 12 amps
and the rated HP of the motor is 5 HP. How many amps will the motor draw if the
technician increases the blower speed from 400 to 475 rpm?
If a pulley of the correct size is placed on the motor to increase the blower
rpm from 400 to 475 rpm, the motor amperage will increase from 8 amps to 13.39
amps. If the motor's FLA is not exceeded in the process, then the change is
permissible without harm to the motor.